Ammonium magnesium phosphate salt liquid fertilizer, preparation method and application thereof

Abstract

This invention discloses a method for preparing magnesium ammonium phosphate (MAP) liquid fertilizer and its application. The steps include: preparing MAP crystals from wastewater by mixing and reacting an aqueous solution containing magnesium ions, ammonium ions, and phosphate ions under pH conditions of 9.0-9.5, followed by settling, washing, and shearing to obtain MAP crystals; subsequently, preparing a suspension by mixing the MAP crystals with deionized water, adding xanthan gum as a dispersant while stirring, subjecting the mixture to high-speed shearing and pH adjustment, and then settling to obtain the MAP liquid fertilizer. This invention uses MAP crystals as raw material and a specific dispersion system to achieve stable and uniform dispersion of MAP crystals in the liquid phase, improving the stability and nutrient distribution uniformity of the liquid fertilizer system, ensuring stable quality and balanced nutrient supply during product storage, transportation, and application. This method forms a novel preparation technology route, filling a gap in the domestic MAP liquid fertilizer preparation field, providing key technical support for its industrialization and large-scale production, and possessing outstanding technical effects and industrial application value.

Description

Technical Field

[0001] This invention relates to the field of liquid fertilizer preparation technology, and in particular to a method for preparing magnesium ammonium phosphate liquid fertilizer and its application. Background Technology

[0002] Magnesium ammonium phosphate (MAP), with the chemical formula MgNH4PO4·6H2O, commonly known as struvite, is a mineral composed of magnesium ions (Mg... 2+ ), ammonium ions (NH4) + ) and phosphate ions (PO4) 3- It is a crystalline compound formed by complexation. Its crystals are orthorhombic and have extremely low solubility in water, with a solubility of approximately 0.02–0.023 g / 100 mL at 25°C. This property allows MAP to slowly release magnesium into the soil as an agricultural fertilizer. 2+ NH4 + PO4 3- This significantly reduces nutrient loss due to leaching or volatilization and improves nutrient utilization efficiency. [Ma Shangyu, Hou Junyou, Wang Yanyan, et al. Research progress on efficient utilization of inorganic nitrogen fertilizer in rice-wheat rotation system [J]. Soil Bulletin, 2021, 52(06):1496-1504. DOI:10.19336 / j.cnki.trtb.2020110901.]

[0003] Liquid fertilizers refer to fertilizer products that exist in liquid form, including solution type and suspension type, and are suitable for fertigation systems. With the vigorous promotion of fertigation technology in my country's agricultural sector, the research and application of liquid fertilizers and related machinery are also gradually developing. The Ministry of Agriculture and Rural Affairs' "Action Plan for Reducing Fertilizer Use by 2025" (Agricultural Development

[2022] No. 8, released on 2022-11-16) clearly proposes to accelerate the demonstration and promotion of fertigation and liquid fertilizers [Zhang Luyun, He Yichuan, Yang Huaijun, et al. Analysis of the Current Status of Liquid Fertilizer Application Machinery and its Relationship with Modern Agriculture [J]. China Agricultural Machinery Chemical Journal, 2021, 42(04):34-40. DOI:10.13733 / j.jcam.issn.2095-5553.2021.04.06.]. To achieve this goal, it is necessary to start with the fertilizer varieties themselves, develop products with higher nutrient utilization rates, and improve the efficiency of liquid fertilizers. Therefore, developing MAP into a liquid fertilizer is expected to combine its slow-release advantages with the application convenience of liquid fertilizers, further meeting the needs of precision agriculture. However, currently, MAP fertilizer products in China are only available in solid form; there are no research reports on MAP liquid fertilizers to date, and no MAP liquid fertilizer products are available on the market.

[0004] MAP is poorly soluble in water, and MAP liquid fertilizer is a type of suspension liquid fertilizer. Suspension liquid fertilizers are prone to problems such as stratification, paste formation, coagulation, bottoming, and low suspension rate during storage. The root cause lies in gravity sedimentation, van der Waals force agglomeration, and Austronesian ripening. Moreover, the high solid content caused by the low solubility of the suspended matter exacerbates these phenomena [Han Zhiwei, Wang Linlin, Liu Zunqi, et al. Research progress on physical stability of suspension fertilizer system [J]. Chinese Soil and Fertilizer, 2024, (06): 252-258+278.]. In the existing technology, the preparation of suspension liquid fertilizer usually requires the use of dispersants, stabilizers, etc., and the homogenization process to maintain the stability of the system. At the same time, preservatives need to be added to inhibit microbial activity. A typical preparation process for liquid fertilizer usually includes steps such as preparing mother liquor, dispersion treatment, homogenization and stirring, and detection [Lu Fei. Preparation of a liquid fertilizer and its application effect on carnation and Aspidistra elatior [D]. Shandong Agricultural University, 2022. DOI:10.27277 / d.cnki.gsdnu.2022.000649.]. In this process, the selection of dispersant is the key, and there are many types of dispersants and stabilizers available, which can be mainly divided into organic polymers (such as xanthan gum, alginic acid, cellulose ether, etc.), small molecule surfactants (such as silicone surfactants, sodium dodecyl sulfate, etc.), inorganic surfactants (such as sodium tripolyphosphate, sodium silicate, etc.), and natural small molecule adjuvants (such as chitosan oligosaccharide, sodium humate, etc.) [Li Jianfeng, Sun Wenguang, Zheng Jiliang. Brief analysis of production technology of suspension liquid fertilizer [J]. Xinjiang Agricultural Science and Technology, 2021, (02):41-43.]. To prevent the system from being damaged by microbial activity, preservatives need to be added. Commonly used preservatives include potassium sorbate, nisin, lysozyme, α-polylysine, and natamycin [Han Zhiwei, Wang Linlin, Liu Zunqi, et al. Research progress on physical stability of suspension fertilizer system [J]. Chinese Soil and Fertilizer, 2024, (06): 252-258+278.]. According to existing technology, grinding solid raw materials to the micron level is a conventional prerequisite for obtaining a stable suspension. This generally requires the use of equipment such as sand mills or colloid mills. The processing time depends on the actual production conditions such as the selection of raw materials and the power of the equipment [Li Jianfeng, Sun Wenguang, Zheng Jiliang. Brief analysis of production technology of suspended liquid fertilizer [J]. Xinjiang Agricultural Science and Technology, 2021, (02): 41-43.]. However, the above schemes are mainly for salts with high solubility or low specific gravity such as monocalcium phosphate and potassium nitrate. They are not applicable to orthorhombic crystals with low solubility and high specific gravity such as MAP.

[0005] Tang Xiaojia et al. applied for a method for preparing controlled-release magnesium hydroxide fertilizer, with the auxiliary agent selected from one or more of sodium polyacrylate, sodium hexametaphosphate, alkyl polyether sulfate, and sodium lignosulfonate [Chinese Invention Patent. A method for preparing controlled-release magnesium hydroxide fertilizer, Patent No.: 202510711741.9]. In this patent, the auxiliary agent is selected from one or more of sodium polyacrylate, sodium hexametaphosphate, etc. That is, additional phosphate or alkali metal salts are required, resulting in high economic costs. Sun Chongqing et al. found that xanthan gum mainly enhances the stability of corn steep liquor-nitrogen phosphorus potassium suspension fertilizer through thickening, steric hindrance, and electrostatic repulsion [Sun Chongqing, Xu Lu, Xu Dehua, et al. Application and performance study of xanthan gum in corn steep liquor-nitrogen phosphorus potassium suspension fertilizer [J]. Chemical and Biological Engineering, 2025, 42(05):37-42.]. In this study, xanthan gum only plays a role in the slight stratification problem of conventional salts and does not involve the specific stabilization design for low-solidity solid particles such as MAP.

[0006] Therefore, developing a new type of MAP liquid fertilizer product with excellent stability and solving the problems of easy water separation and sedimentation of MAP liquid fertilizer is of great significance to meeting the needs of precision fertilization in modern agriculture. Summary of the Invention

[0007] This invention provides a method for preparing magnesium ammonium phosphate liquid fertilizer and its application by optimizing the particle size distribution of raw materials, selecting a specific dispersant, and constructing a stabilization and regulation mechanism for the MAP suspension system. This overcomes the problems of poor stability, easy water separation, and sedimentation of liquid fertilizers.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows: This invention provides a method for preparing magnesium ammonium phosphate liquid fertilizer, comprising the following steps: Preparation of MAP crystals from wastewater: Under pH conditions of 9.0-9.5, an aqueous solution containing magnesium ions, ammonium ions, and phosphate ions is mixed and reacted, then allowed to stand, washed, and sheared to obtain MAP crystals; Preparation of suspension: MAP crystals are mixed with deionized water, xanthan gum is added as a dispersant under stirring, followed by high-speed shearing treatment, pH is adjusted to 4-11, and after standing, the MAP liquid fertilizer is obtained; The amount of xanthan gum added is 0.05%-5% of the total mass of the system, preferably 0.2%.

[0009] The magnesium ions are derived from MgCl2·6H2O, MgSO4, MgO or Mg(OH)2, the ammonium ions are derived from NH4Cl or (NH4)3PO4, and the phosphate ions are derived from Na2HPO4, NaH2PO4, K2HPO4, KH2PO4, H3PO4 or (NH4)3PO4.

[0010] Furthermore, the amount of MAP crystals added is 5%-70% of the total mass of the system, preferably 20%.

[0011] Furthermore, the pH of the magnesium ammonium phosphate liquid fertilizer is 8-9.

[0012] Furthermore, the high-speed shearing speed is 30,000 rpm, and the shearing time is 10-30 minutes, preferably 20 minutes.

[0013] Furthermore, the temperature of the magnesium ammonium phosphate liquid fertilizer is 10℃-80℃.

[0014] Furthermore, the MAP crystals have a particle size distribution of 1100-1700 nm and a median diameter of 1376.67 nm.

[0015] In another aspect, the present invention provides a liquid fertilizer of magnesium ammonium phosphate prepared according to the preparation method described above, wherein the liquid fertilizer of magnesium ammonium phosphate uses xanthan gum as a single dispersant, so that magnesium ammonium phosphate crystals can be stably suspended and dispersed in the system. The precipitation rate of the magnesium ammonium phosphate liquid fertilizer after standing for 14 days was 0, and the viscosity was 60.146±0.902 mpa·s.

[0016] Furthermore, the amount of xanthan gum added is 0.2% of the total mass of the system, and the amount of MAP crystals added is 20% of the total mass of the system.

[0017] Furthermore, the magnesium ammonium phosphate liquid fertilizer may be supplemented with functional adjuvants, which may be selected as a 0.1% sodium benzoate preservative or a 15% glycerol antifreeze.

[0018] In another aspect, the present invention provides the application of the aforementioned magnesium ammonium phosphate liquid fertilizer in an integrated agricultural water and fertilizer system.

[0019] The beneficial effects of this invention are: This invention discloses a method for preparing magnesium ammonium phosphate (MAP) liquid fertilizer. Using MAP crystals as raw material and a specific dispersion system, the method achieves stable and uniform dispersion of MAP crystals in the liquid phase, significantly improving the system stability and nutrient distribution uniformity of the liquid fertilizer. This ensures stable product quality and balanced nutrient supply during storage, transportation, and application. This method forms a new technical route for MAP liquid fertilizer preparation, filling a technological gap in the domestic MAP liquid fertilizer preparation field. It enriches the product forms and application methods of MAP fertilizers, providing key technical support for the industrial production and large-scale application of MAP liquid fertilizer, and possesses outstanding technical effects and industrial application value. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 Comparison of the appearance of MAP liquid fertilizer suspensions after standing for 14 days under different stabilizer addition conditions; among them, Figure 1 A is a morphological image of a MAP liquid fertilizer suspension prepared with xanthan gum (0.2%), sodium carboxymethyl cellulose (0.2%), MAP (20%), a shearing time of 20 minutes, a pH of 8, and a preparation temperature of room temperature, after standing for 14 days. Figure 1 Image B shows the appearance of a MAP liquid fertilizer suspension prepared at room temperature with 0.2% xanthan gum, 20% MAP, a shearing time of 20 minutes, a pH of 8, and a preparation temperature of room temperature, after standing for 14 days. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] The mechanism involved in this invention: Xanthan gum, an anionic polysaccharide, has polymer chains that intertwine in aqueous solution through hydrogen bonding, hydrophobic interactions, and electrostatic interactions to form a dense and stable three-dimensional network structure. This structure significantly increases the viscosity of the system and generates a strong steric hindrance effect, providing stable structural support for the suspension system. Based on these inherent properties of xanthan gum, this invention uses it as a single dispersant in the preparation of MAP liquid fertilizer. Its polymer chains can be rapidly dispersed in the aqueous system. On the one hand, the three-dimensional network structure physically encapsulates the MAP crystalline particles, preventing collisions, aggregation, and sedimentation between particles. On the other hand, the anionic groups on the xanthan gum molecular chains can generate specific electrostatic adsorption with the surface of MAP particles, further enhancing the dispersibility of MAP particles in the aqueous phase and establishing a dual stabilization mechanism of steric hindrance and electrostatic adsorption.

[0024] With xanthan gum added at a concentration of 0.05%-5% (preferably 0.2%) of the total system mass, this dosage allows xanthan gum to fully utilize its structural characteristics and dispersing efficiency. It can increase the overall viscosity of the system through its three-dimensional network structure, slowing down the sedimentation rate of MAP particles, and also regulate the surface charge distribution of MAP particles through electrostatic adsorption, preventing particle aggregation due to charge imbalance. This achieves long-term stable suspension and dispersion of MAP crystals in the aqueous system. Based on the above mechanism, this invention discloses a MAP liquid fertilizer, using MAP as the core nutrient and a specific amount of xanthan gum as a dispersant. Through the specific interaction between xanthan gum and MAP particles, a dual stabilization mechanism is constructed, effectively maintaining the homogeneity of the MAP liquid fertilizer system. This ensures uniform nutrient distribution during storage, transportation, and application, while also guaranteeing the stability of the core MAP nutrient, preventing nutrient loss or efficacy reduction due to sedimentation or water separation. This provides clear mechanistic support for the industrial production and practical application of MAP liquid fertilizer, further demonstrating the innovation and feasibility of the invention's technical solution.

[0025] Experimental materials: Reagents: Magnesium aluminum silicate (Maclean, M875683), xanthan gum (Aladdin, G104873), sodium carboxymethyl cellulose (Maclean, viscosity 3000-5000 mpa·s), NH4Cl, Na2HPO4, MgCl2·6H2O (all analytical grade). Equipment: Shear mixer (model D2526Pro), Anton Paar MCR72 rheometer.

[0026] Experimental methods: Example 1: Investigating the effect of different dispersants on the stability of magnesium ammonium phosphate (MAP) liquid fertilizer. A method for preparing magnesium ammonium phosphate liquid fertilizer includes the following steps: Preparation of MAP crystals from wastewater: Weigh 21.8 g NH4Cl and 57.85 g Na2HPO4, add them to a beaker, and add 300 mL of pharmaceutical wastewater sample (hereinafter referred to as the water sample). Stir at 150 rpm and 30℃ until fully dissolved. Weigh 82.84 g MgCl2·6H2O, add it to another beaker, add 100 mL of the water sample, and stir at 150 rpm and 30℃ until fully dissolved. Slowly add the MgCl2·6H2O solution to the NH4Cl and Na2HPO4 mixture at 30℃ with a rotor stirrer at 150 rpm, adjusting the pH while adding, maintaining the pH at 9.0-9.5. After standing for 1 day, discard the supernatant. Wash the crystals twice with deionized water. Place the MAP crystals in a shear press, add 400 mL of deionized water, and shear and stir at 30,000 rpm and room temperature for 20 minutes to break up large crystal aggregates. Further precipitation yielded approximately 100 g of MAP crystals with a dry weight.

[0027] The particle size of the obtained MAP crystals was determined using a Malvern Instruments nanoparticle size and zeta potential analyzer (model ZS-90). The main particle size of the MAP crystals was in the range of 1100–1700 nm, with a median diameter of 1376.67 nm. If commercial magnesium ammonium phosphate is used to prepare MAP liquid fertilizer, the magnesium ammonium phosphate needs to be ground to a particle size of less than 5 μm.

[0028] 2. Preparation of MAP liquid fertilizer (MAP suspension) Measure 400 mL of deionized water and add it to a beaker. Add 100 g of the MAP crystals prepared above (MAP addition amount is 20%). Slowly add the dispersant while stirring with a rotor stirrer at 150 rpm. This experiment is divided into two types: using a single dispersant and using a combination of two dispersants. Among them, the single dispersant experimental schemes are numbered 1-3: No. 1 uses only magnesium aluminum silicate as a dispersant, with an addition amount of 2% of the total mass; No. 2 uses only xanthan gum as a dispersant, with an addition amount of 0.2% of the total mass; No. 3 uses only sodium carboxymethyl cellulose as a dispersant, with an addition amount of 0.2% of the total mass. The combined dispersant experimental schemes are numbered 4-6: No. 4 uses a combination of magnesium aluminum silicate and sodium carboxymethyl cellulose, with addition amounts of 2% and 0.2% of the total mass, respectively; No. 5 uses a combination of xanthan gum and magnesium aluminum silicate, with addition amounts of 0.2% and 2% of the total mass, respectively; No. 6 uses a combination of sodium carboxymethyl cellulose and xanthan gum, with the addition amount of both dispersants being 0.2% of the total mass. The mixture was then transferred to a shear mixer and sheared at 30,000 rpm at room temperature for 20 minutes. The pH was then adjusted to 8 using a 1 mol / L KOH solution, and the mixture was allowed to stand for 30 minutes. Finally, it was sheared and stirred at 30,000 rpm at room temperature for 10 minutes, and allowed to stand for 30 minutes to obtain the MAP liquid fertilizer (MAP suspension).

[0029] 3. Determination of MAP suspension precipitation rate 3.1 Method for determining the MAP liquid fertilizer release rate: Pour 30 mL of the prepared suspension into a 50 mL colorimetric tube. After standing for a certain period of time, measure the height of the precipitated water. The percentage of the height of the precipitated water to the total height is the precipitation rate.

[0030] 3.2 Method for determining the viscosity of MAP liquid fertilizer: Using an Anton Paar MCR72 rheometer at room temperature, with a coaxial cylindrical clamp, the deformation and flow properties of the samples were measured in rotational shear and oscillating shear modes.

[0031] Flow curve test (rotation mode): ① At a constant shear rate of 200 s -1 Shear for 60 seconds to ensure uniform sample dispersion; ② Let stand for 30 seconds; ③ Measure the upward curve. Shear rate is between 2 and 500 s⁻¹. -1 The range was expanded to include 35 measurement points, with a sampling time of 2 seconds. ④ Hold for 500 seconds -1 The settings remain unchanged, with 30 measurement points and a sampling time of 2 seconds. ⑤ Measure the downward curve. The shear rate is between 500 and 2 s.-1 The range is reduced, 35 measurement points are set, and the sampling time is 2 seconds; Each sample was measured three times, and the average value was taken.

[0032] 4. Measurement results: The results are shown in Table 1 and Table 2.

[0033] Table 1. Results of the effect of different single dispersants on the stability of MAP liquid fertilizer

[0034] Note: In Table 1, the experiment was conducted at a MAP addition of 20%, a shearing time of 20 minutes, a pH of 8, and a preparation temperature of room temperature (20℃-25℃).

[0035] As shown in Table 1, xanthan gum exhibited the best results, with a 0% precipitation rate and a viscosity of 60.146 mPa·s for the MAP liquid fertilizer after one day. Furthermore, the MAP liquid fertilizer prepared using xanthan gum as the sole dispersant maintained a 0% precipitation rate even after 14 days of standing, demonstrating excellent long-term storage stability. The experimental results indicate that compared to commonly used dispersants such as magnesium aluminum silicate and sodium carboxymethyl cellulose, xanthan gum most effectively prevents the sedimentation and stratification of MAP particles while providing suitable viscosity, making it a key additive for achieving high stability in MAP liquid fertilizer.

[0036] Table 2. Results of the effect of compound dispersants on the stability of MAP liquid fertilizer

[0037] Note: In Table 2, the experiment was conducted at a MAP addition of 20%, a shearing time of 20 minutes, a pH of 8, and a preparation temperature of room temperature (20℃-25℃).

[0038] Table 2 shows that the stability of the xanthan gum and magnesium aluminum silicate compound system is better than that of the sodium carboxymethyl cellulose and magnesium aluminum silicate compound system, the magnesium aluminum silicate and xanthan gum compound system, and the sodium carboxymethyl cellulose and xanthan gum compound system. Specifically, the sodium carboxymethyl cellulose and magnesium aluminum silicate compound system showed an 83.3% precipitation rate within one day, indicating rapid stratification and instability, and extremely poor stability. After combining sodium carboxymethyl cellulose and xanthan gum, the precipitation rate of the suspension reached 6.7% within one day, and increased to 81.7% and 83.3% at 7 and 14 days, respectively, showing significantly worse stability than the xanthan gum single system. The morphology of the MAP liquid fertilizer suspension after 14 days of standing is shown in the figure below. Figure 1 ( Figure 1As shown in A). Furthermore, xanthan gum as a dispersant alone is more effective than its combination with magnesium aluminum silicate: when used alone, no water precipitates after 14 days of standing; however, the xanthan gum and magnesium aluminum silicate combination system shows significant water precipitate after only 7 days. This indicates that the introduction of magnesium aluminum silicate does not produce a synergistic effect; instead, it interferes with the stable three-dimensional network structure formed by the xanthan gum system alone, leading to a decrease in its steric hindrance effect and thus inferior stability compared to the xanthan gum system alone. This result further confirms that for the MAP liquid fertilizer system of this invention, using xanthan gum alone is a better way to achieve optimal stability.

[0039] Example 2: Investigating the effect of different amounts of xanthan gum added on the stability of MAP liquid fertilizer Compared with Example 1, this example sets different xanthan gum addition amounts (0.05%, 0.1%, 0.2%, 0.3%, 1%, 3%, 5%), and prepares MAP liquid fertilizer according to the preparation method of Example 1. The precipitation rate of the prepared MAP liquid fertilizer is measured and compared with that of Example 1 with a xanthan gum addition amount of 0.2%. The results are shown in Table 3.

[0040] Table 3. Stability of MAP suspensions with different xanthan gum additions

[0041] Note: In Table 3, the experiment was conducted at a MAP addition of 20%, a shearing time of 20 minutes, a pH of 8, and a preparation temperature of room temperature.

[0042] As shown in Table 3, adding 0.2% xanthan gum by total mass of the system yielded the best results (Example 1), with a precipitation rate of 0% after 1 day and 0% after 14 days, and a measured viscosity of 60.146 mPa. The system exhibits both suitable viscosity and long-term suspension stability. When xanthan gum is added at 0.05%, an effective spatial network support cannot be formed, making it difficult for MAP particles to disperse stably, resulting in a 1-day precipitation rate of 81.7% and extremely poor stability. At 0.1% xanthan gum addition, the 1-day precipitation rate is relatively high at 80%. When the xanthan gum addition is higher than 0.3%, no precipitation is observed within 1 and 7 days, indicating good suspension performance; however, the precipitation rate at 14 days is 56.3%, which is inferior to Example 1 with 0.2% xanthan gum addition. Considering both short-term and long-term stability and system viscosity matching, 0.2% xanthan gum addition is the optimal addition ratio to achieve high suspension stability and suitable viscosity for MAP liquid fertilizer.

[0043] Example 3: Investigating the effect of different MAP addition amounts on the stability of MAP liquid fertilizer Compared with Example 1, this example sets different MAP addition amounts (5%, 10%, 30%, 50%, 70%, the addition amount is the mass percentage of the total system mass), and prepares MAP liquid fertilizer according to the preparation method of Example 1. The precipitation rate of the prepared MAP liquid fertilizer is measured and compared with that of Example 1 with a MAP addition amount of 20%. The results are shown in Table 4.

[0044] Table 4. Suspension stability at different MAP addition levels

[0045] Note: In Table 4, the experiment was conducted at a xanthan gum addition of 0.2%, a shearing time of 20 minutes, a pH of 8, and a preparation temperature of room temperature.

[0046] As shown in Table 4, the best effect was achieved when the MAP addition amount was 20% (Example 1), with a 0% precipitation rate after 1 day and a measured viscosity of 60.146 mPa·s. Furthermore, it exhibited outstanding long-term stability, with no precipitation observed even after 14 days of standing. This indicates that at this addition amount, the MAP particles and xanthan gum dispersion system achieved optimal compatibility. The appropriate particle concentration allows xanthan gum to fully exert its dispersing effect, maintaining a suitable viscosity in the system. This not only prevents particle aggregation and sedimentation but also ensures the uniformity of the liquid fertilizer, achieving stable nutrient retention. When the MAP addition was 5%, although there was no precipitation in the short term (1 day), the precipitation rate reached as high as 93.3% after 7 days. This was because the MAP particle concentration was too low to effectively synergize with the stable system built by xanthan gum. The xanthan gum network structure lacked sufficient particle support, leading to a rapid decline in system stability. This failed to meet the stability requirements during the storage and transportation of liquid fertilizers. Furthermore, the excessively low nutrient concentration also affected the fertilizer application effect. When the MAP addition was 10%, the precipitation rate reached 62.5% after 7 days and rose to 86.7% after 14 days. This indicates that although the particle concentration increased compared to 5%, and stability was improved to some extent, it still did not reach the critical concentration required to form a long-lasting stable system. The xanthan gum network structure did not receive sufficient support, and severe stratification still occurred during long-term storage. When the MAP addition exceeded 20%, the system stability began to decline. At an addition of 30%, 3.3% precipitation occurred within 1 day, indicating that the particle concentration was approaching the critical point of xanthan gum's dispersibility. The problems became more pronounced when the addition amount increased to 50% and 70%. At 50%, 3.3% precipitation occurred within one day, and the precipitation rate increased significantly to 36.7% after 14 days. At 70%, clumping occurred in the initial stage (within one day). This indicates that excessive MAP particles exceeded the dispersion carrying capacity of xanthan gum, increasing the probability of particle collisions, leading to agglomeration and sedimentation, resulting in insufficient long-term stability. Furthermore, excessive particles can reduce the fluidity of the system, increasing the difficulty of application. In summary, a MAP addition amount of 20% achieves the optimal balance between system stability, suitable viscosity, and nutrient concentration, representing the optimal addition ratio that considers both product performance and practical application needs. Example 4: Investigating the effect of different shear times on the stability of MAP liquid fertilizer. Compared with Example 1, this example sets different shearing times (10 minutes and 30 minutes), and prepares MAP liquid fertilizer according to the preparation method of Example 1. The precipitation rate of the prepared MAP liquid fertilizer is measured and compared with that of Example 1 with a shearing time of 20 minutes. The results are shown in Table 5.

[0047] Table 5. Stability of liquid fertilizer at different shear times

[0048] Note: In Table 5, the experiment was conducted at a MAP addition of 20%, a xanthan gum addition of 0.2%, a pH of 8, and a preparation temperature of room temperature.

[0049] As shown in Table 5, a shearing time of 20 minutes yielded the best results, with a 0% precipitation rate after one day and a measured viscosity of 60.146 mPa·s. At this time, the shearing was sufficient without damaging the system structure, allowing for uniform mixing of MAP particles and xanthan gum dispersant, ensuring system stability. A shearing time of 10 minutes resulted in insufficient shearing, uneven particle dispersion, and a precipitation rate of 63.3% after one day, indicating poor stability. A shearing time of 30 minutes led to excessive shearing, damaging the xanthan gum dispersion system, with a precipitation rate of 9.1% after one day, and stability lower than the optimal group. In conclusion, 20 minutes is the optimal shearing time, balancing sufficient shearing with system integrity.

[0050] Example 5: Investigating the effect of different pH values ​​on the stability of MAP liquid fertilizer Compared with Example 1, this example sets different pH values ​​(4, 5, 6, 7, 9, 10, 11) to prepare MAP liquid fertilizer according to the preparation method of Example 1. The precipitation rate of the prepared MAP liquid fertilizer is measured and compared with that of Example 1 with pH 8. The results are shown in Table 6.

[0051] Table 6. Stability of liquid fertilizer at different pH levels

[0052] Note: In Table 6, the experiment was conducted at a MAP addition of 20%, a xanthan gum addition of 0.2%, a shearing time of 20 minutes, and a preparation temperature of room temperature.

[0053] As shown in Table 6, the prepared liquid fertilizer exhibits the best stability at a pH of 8-9, with a 1-day precipitation rate of 0 and a measured viscosity of 60.146 mPa·s. Below pH 6, the system separates into layers within 1 day of standing; above pH 9, the system separates into layers within 1 day of standing. In conclusion, a pH of 8-9 represents the optimal acidity / alkalinity for the system.

[0054] Example 6: Investigating the effect of different temperatures on the stability of MAP liquid fertilizer Compared with Example 1, this example sets different temperatures (10℃, 20℃, 40℃, 60℃, 80℃) to prepare MAP liquid fertilizer according to the preparation method of Example 1. The precipitation rate of the prepared MAP liquid fertilizer is measured and compared with that at room temperature in Example 1. The results are shown in Table 7.

[0055] Table 7. Stability of liquid fertilizer at different temperatures

[0056] Note: In Table 7, the experiment was conducted at the following conditions: MAP addition of 20%, xanthan gum addition of 0.2%, shearing time of 20 minutes, and pH of 8.

[0057] As shown in Table 7, the MAP system maintains stable reaction characteristics over a wide temperature range, and temperature fluctuations do not substantially interfere with the system. Considering the stringent requirements for energy efficiency control and cost-effectiveness in industrial production, it is preferable to control the preparation temperature within the room temperature range. Figure 1 Figure B shows the appearance of the liquid fertilizer suspension after standing for 14 days (right) when the xanthan gum content is 0.2%, the MAP content is 20%, the shearing time is 20 minutes, the pH is 8-9, and the preparation temperature is room temperature (Example 1).

[0058] The above results indicate that the system exhibits the best stability under the following conditions: MAP addition of 20%, xanthan gum addition of 0.2%, shear time of 20 minutes, pH of 8-9, and preparation temperature of room temperature. Based on this, the formulation can be further optimized by introducing functional additives, such as adding 0.1% sodium benzoate as a preservative and 15% glycerol as an antifreeze agent, to meet the comprehensive requirements of product storage stability and low-temperature adaptability.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a liquid fertilizer of magnesium ammonium phosphate, characterized in that, Includes the following steps: Under pH conditions of 9.0-9.5, aqueous solutions containing magnesium ions, ammonium ions, and phosphate ions are mixed and reacted, then allowed to stand, washed, and sheared to obtain magnesium ammonium phosphate (MAP) crystals. The MAP crystals are mixed with deionized water, and xanthan gum is added as a dispersant under stirring conditions. Then, high-speed shearing is performed, the pH is adjusted to 4-11, and after standing, the magnesium ammonium phosphate liquid fertilizer is obtained. The amount of xanthan gum added is 0.05%-5% of the total mass of the system.

2. The preparation method according to claim 1, characterized in that, The amount of magnesium ammonium phosphate (MAP) crystals added is 5%-70% of the total mass of the system.

3. The preparation method according to claim 1, characterized in that, The pH of the magnesium ammonium phosphate liquid fertilizer is 8-9.

4. The preparation method according to claim 1, characterized in that, The high-speed shearing speed is 30,000 rpm, and the shearing time is 10-30 minutes.

5. The preparation method according to claim 1, characterized in that, The temperature of the magnesium ammonium phosphate liquid fertilizer is 10℃-80℃.

6. The preparation method according to claim 1, characterized in that, The particle size distribution of the magnesium ammonium phosphate (MAP) crystals is 1100-1700 nm, with a median diameter of 1376.67 nm.

7. The ammonium magnesium phosphate liquid fertilizer prepared by the method according to claim 1, characterized in that, The magnesium ammonium phosphate liquid fertilizer uses xanthan gum as a single dispersant, which enables the magnesium ammonium phosphate crystals to be stably suspended and dispersed in the system. The precipitation rate of the magnesium ammonium phosphate liquid fertilizer after standing for 14 days was 0, and the viscosity was 60.146±0.902 mPa·s.

8. The application of the magnesium ammonium phosphate liquid fertilizer according to claim 7 in an integrated agricultural water and fertilizer system.

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